We usually imagine the death of a star as a violent and explosive process. And in some cases it is like that, because some stars end their days exploding in the form of a supernova , surpassing in luminosity for a few moments the galaxy of which they are a part. However, this ending is only accessible to the most massive stars and the vast majority of stars will not end up exploding, but will have a relatively quieter death. These stars, including our Sun, will eventually form a white dwarf, which consists of the stellar core left over after shedding its outer layers during its giant star phase. These stars shine by inertia, because they were originally a nucleus at a temperature of millions of degrees and will continue to radiate their energy for billions of years to come. But inside it, nuclear fusion processes no longer take place , which are what keep conventional stars alive and shining. This is why we consider, perhaps too poetically, that white dwarfs are dead stars .
A recent study led by J. Farihi of University College London has discovered a disk of material orbiting a white dwarf in our galaxy. This disk presents a certain structure and periodicity in its orbit whose simplest and most plausible explanation would be the presence of a planet that stabilized said disk . This study used observations made from the La Silla observatory in Chile and a Spanish researcher from the University of La Laguna, Paula Izquierdo, participated in it.
To understand the significance of the discovery, let’s first understand what exactly a white dwarf is. Stars spend most of their lives on what is known as the main sequence , fusing first hydrogen and later, and if they have enough mass, heavier elements. The Sun is in the middle of its time in this state right now , with 4 to 5 billion years remaining. When hydrogen, which is the main component of all stars (and the universe as a whole, for that matter) is depleted , changes take place inside the star . The core contracts , increasing the pressures and temperatures in this region, making helium fusion possible . As a consequence of this increase in temperatures (reaching millions of degrees), the outer layers begin to expand and cool . In doing so, its color will change from the original yellowish or whitish towards more orange and reddish tones, becoming a red giant star in the process .
There will come a time when these layers will have expanded so much and will receive such an amount of radiation from the inner regions that they will end up being thrown out into the interstellar medium , forming a nebula around the star. When these layers have disappeared, what will remain will be a white dwarf , which may accumulate half the mass of the original star in a volume comparable to that of the Earth.
The expansion of the outer layers of the star reaches such proportions that it is believed that when the Sun reaches this stage of its development, it will engulf Mercury and Venus and perhaps even our planet , multiplying its size thousands of times. In addition, the strong solar winds and the expulsion of their outer layers will undoubtedly affect the gas giants , causing them to lose much of their mass. This being the case we could not expect the solar system to survive the death of the Sun unchanged . To be sure, little will survive of the three planets that are close enough to be gobbled up when the time comes. That is why the discovery published in the mentioned article and other similar discoveries made in recent years are especially surprising.
The structures discovered around the white dwarf WD 1054–226 and the planet that would guarantee its stability are too close to the star to have occupied that same position before the transformation into a white dwarf . But what exactly have they discovered? In this study, a kind of dust disk has been observed around the central white dwarf, but this disk was not uniform , but rather had groups of matter that were able to partially hide the star , diminishing the brightness detected from Earth. All these clouds of gas and dust orbit the star every 25 hours . With 65 of them, this meant that one of them occluded the white dwarf every 23 minutes.
This behavior and the observed regularity are analogous to what is observed in gas giant ring systems , in which some moons , orbiting between the rings themselves, are capable of creating periodic disturbances in the rings. These moons are known as shepherd moons, as if they are herding a flock of dust clouds.
As if this discovery were not exciting enough, the planet causing all this phenomenon would orbit within the habitable zone of the star , an area in which there could be liquid water on its surface. Of course, this habitable zone would be as new as the white dwarf itself, and not earlier, so if life had arisen in a plant with these characteristics, it would be a relatively recent life . Future research will delve into the characteristics of this possible planet, should it join the more than 5,000 exoplanets discovered to date.
Farihi et al, 2022, Relentless and complex transits from a planetesimal debris disc, Monthly Notices of the Royal Astronomical Society, 511, 2, April 2022, https://doi.org/10.1093/mnras/stab3475